During a scanning performed by a computed tomography (CT) scanner equipped with anti-scatter grids (ASGs), the displacement of the focal point of the radiation source of the CT scanner may cause a portion of the radiation emitted by the radiation source intended to be received by the detector of the CT scanner blocked by the ASGs, causing a reduction of the quality of the image generated based on the scanning. Various hardware related (e.g., focus point tracing) or software related (e.g., image post-processing) techniques may be adopted for compensating the image quality reduction. However, the displacement of the focal point needs to be determined for most of these techniques. There is a need for a method of low cost and high reliability for determining the displacement of the focal point during the scanning of the CT scanner.